The continuing development and widespread implementation of wireless/radio communication systems, such as wireless PAN (personal area network), wireless LAN (local area network), wireless WAN (wide area network), cellular networks, etc., is driving the market demand for high-performance, highly-integrated and low-power, low cost solutions for on-chip radio communication systems that operate at millimeter-wave frequencies. For millimeter wave applications, integrated devices (e.g., integrated transmitter, receiver, transceiver systems) are typically fabricated using GaAs or InP semiconductor technologies, as such technologies can provide the speed and power that is needed for applications such as personal-area networks or automotive radars. It has been demonstrated that silicon germanium (SiGe) process technologies, for example, are well positioned to provide such solutions for highly integrated radio communication circuits.
Transistor amplifiers (such as power amplifiers) are essential components in radio communications systems that are used to amplify signals to desired power levels for delivery to a load. In general, various on-chip impedance matching techniques can be used for tuning integrated transistor amplifiers to achieve high output power and efficient operation for a given class of amplifier. Conventional impedance matching or tuning circuits generally include resonant LC circuits (which are implemented using on-chip capacitors and inductors), and integrated transformers (e.g., coupled-wires). In conventional amplifier designs, monolithic on-chip transformers and on-chip capacitors are typically used to design tuning circuits to achieve a desired bandwidth and efficiency at operating frequencies up to a few tens of GHz.
By way of example, FIG. 1A is a generic schematic illustration of a conventional transformer-coupled power amplifier circuit. The power amplifier (10) comprises a transistor amplifier (11) that drives a load impedance (14) via an impedance matching circuit, which comprises a tuning (shunt) capacitor (12) and transformer (13). The shunt tuning capacitor (12), which is connected in parallel across the input terminals (primary winding) of the transformer (13), is selected to tune the reactive component of the transformer input impedance to a desired value appropriate to provide the required drain/collector impedance for the given class of amplifier.
In FIG. 1A, the transformer (13) is represented by a low-frequency equivalent circuit comprising an ideal transformer (13a) and lumped inductors (13b) and (13c). The ideal transformer (13a) has no impedance by itself (ignoring primary inductance/resistance) and simply reflects the impedance load (14) on the secondary back to the primary. The transformer does, however, have a primary inductance (Lp), which has a direct effect on the low frequency response of the transformer (as explained below). The shunt inductor (13b) represents the transformer mutual inductance (or magnetizing inductance) as seen on the primary side, and has a value of k2Lp, where k is the coupling factor and Lp is the primary inductance. The series inductor (13c) represents transformer leakage inductance as seen on the primary side and has a value of (1−k2) Lp. The leakage inductance is caused by incomplete magnetic coupling between the primary and secondary windings. The low-frequency equivalent circuit model of FIG. 1A omits the parasitic transformer capacitance (which is connected in parallel across inductor (13b)) since the impedance of such parasitic capacitor becomes sufficiently high at low frequencies to permit ignoring its effect.
As noted above, the primary inductance (Lp) has a direct effect on the low frequency response of the transformer. The low frequency −3 dB cutoff point fpk can be determined as follows: fpk=Z/(2 pi Lp sqrt(1−k2)), wherein Z is the primary source impedance (which is the reflected impedance in parallel with the source impedance presented by the transistor T1) and wherein Lp is the primary inductance and k the transformer coupling factor as noted above. The low frequency response of the transformer can be improved by increasing the primary inductance Lp (which means a larger structure (core) and/or more turns on the primary). The high frequency limit of the transformer is affected by the leakage inductance (13c) and the distributed capacitance of the inductor (13b), which together form a second order low pass filter, as well as other factors. As the transformer primary inductance Lp is increased, the low frequency response improves but at the expense of a higher and distributed capacitance, which limits the high frequency response.
For on-chip applications, transformers are typically constructed using coupled wires. For example, a conventional on-chip transformer structure comprises two wires (metallic lines) with the same windings on each side, which is referred to as a 1:1 transformer or simply coupled-wires. In particular, an on-chip transformer may be constructed having a first elongated conductor (primary) and a second elongated conductor (secondary) which that are disposed parallel to each other and on a same layer (coplanar). The conductors are patterned from a metal layer that is formed on the substrate surface, and then encapsulated in a dielectric or insulating layer.
Although on-chip transformers (coupled-wires) can be used in transistor amplifier impedance matching networks as depicted in FIG. 1A, implementation becomes more problematic as the operating frequencies are increased to the millimeter wave range. For example, to operate the transformer in the millimeter-wave frequency range, the primary inductance Lp has to be reduced substantially, which in turn, requires the value of additional tuning capacitor (12) to be extremely small. However, at millimeter-wave frequencies, it is increasingly difficult to place small on-chip capacitors in parallel with the inductors to provide the optimum impedance.
Moreover, tuned circuits that are formed using on-chip transformers and capacitors may not have the bandwidth required for the amplifier. Indeed, narrow bandwidths are typically achieved when tuned circuits are formed using conventional coupled wire transformers (such as described above) because such transformers typically exhibit poor electrical performance e.g., low coupling, k=0.06 and high loss, especially with high frequency applications.
Indeed, for lossy substrates such as silicon, the capacitive coupling between the primary and secondary metal lines and the substrate can result in increased power dissipation. If the metal lines are reduced in width to limit such capacitive coupling, the resistance of the metal line increases (e.g., via skin effect) resulting in increased power dissipation.
Moreover, conventional transformer designs do not have well-defined return paths for closed environment EM conditions, which results in the electrical performance being more sensitive to surrounding metallic components. Accordingly, integrated circuit coplanar transformer devices are typically used at lower frequencies where lower coupling factors, losses due to the skin effect, and inaccuracies caused by model to hardware discrepancies can be tolerated.